1. Field of the Invention
The present invention relates to a projection type image display apparatus EL element such as a projector, etc.
2. Description of Related Art
A conventional projection type display normally uses a liquid crystal panel or micromirror device as a light modulation device for switching, projects a light pattern selected by controlling transmission, shielding or deflection of light onto a screen, and thereby displays the image on the screen.
When a liquid crystal panel or micromirror device is used as a light modulation device on the above-described display, it is assumed that shielded light is regarded as unnecessary energy and absorbed by a polarization element or light absorption medium and eliminated. Therefore, to brighten a displayed image, a metal halide or high-pressure mercury lamp is used as a light source, but there are additional problems that a high voltage needs to be used as a light source voltage and the light source produces high heat.
As a method for fundamentally solving such low efficiency of energy usage, there are proposals disclosed in Japanese Patent Application Laid-Open No. H11(1999)-67448(corresponding to U.S. Pat. No. 6,259,423) and Japanese Patent Application Laid-Open No. 2000-66301. In the above-described two proposals, an electroluminescence (EL) panel (hereinafter referred to as “organic EL panel”) on which organic electroluminescence elements (hereinafter referred to as “organic EL elements”) are arranged in matrix form is used, light emission of each organic EL element of this organic EL panel is driven based on image signals and light from each organic EL element is projected onto a display target object through a projection optical system to display an image.
Since the organic EL element is a self-light-emitting element, there is no need for any separate illumination light source. Furthermore, the organic EL panel can perform light emission based on image information, and therefore there is no need for a transmission type liquid crystal panel, etc. Therefore, it is possible to use the light obtained for displays effectively. The organic EL element can therefore easily display images with lower power without generating unnecessary light energy. Furthermore, since the organic EL panel alone can output images, the apparatus structure is simple and it is easy to reduce the size and weight of the apparatus.
On the other hand, with respect to the characteristic of a projection lens, the above-described Japanese Patent Application Laid-Open No. H11(1999)-67448 and Japanese Patent Application Laid-Open No. 2000-66301 do not disclose any projection lens which is telecentric toward the light source, which captures a light beam in the direction of the normal line of light beams emitted from emission pixels with emission directivity having maximum emission intensity in the direction perpendicular to the emitting surface of the organic EL element at the center of an incident pupil of the projection lens.
In the case of a liquid crystal panel, in order to prevent contrast of image information from reducing, it is necessary to project images using a projection lens which is telecentric toward the liquid crystal light panel. Furthermore, when a micromirror device is used as a light modulation device, the deflection angles of the respective micromirrors are all the same, therefore to capture the light modulated and reflected it is necessary to project images using a projection lens which is telecentric toward the micromirror device. That is, in these cases, projection lenses characterized by being telecentric toward the modulation panel are necessarily used.
Furthermore, as disclosed in the above-described Japanese Patent Application Laid-Open No. H1(1999)-67448, in the case where a light resonance structure made up of a metal electrode in the organic EL element structure and a multilayer dielectric interference reflecting mirror provided outside a transparent electrode facing this metal electrode is used to enhance directivity of the emission characteristic of light, the emission wavelength changes depending on the angle of emission, and therefore it is essential to capture light using a projection lens which is telecentric toward the organic EL element.
Here, when organic EL element having no such light resonance structure is used as an image modulation light-emitting element, a system which captures light using the projection lens which is telecentric toward the organic EL element may be considered as an ideal system for the purpose of capturing a maximum amount of light emitted from the element.
However, when the actual projection system is constructed, the above-described projection lens having a telecentric characteristic has difficulty in keeping the vignetting factor of the pupil at 100% from the standpoint of the design as the object height from the optical axis, that is, the distance of the light-emitting position of the organic EL element from the optical axis increases. For this reason, a design that decreases the vignetting factor as the object height increases is adopted in pursuit of compactness and low cost of lenses.
With regard to a reduction of vignetting factor, that is, increase of vignetting, it is an actual situation that when the field angle of a projection type image display apparatus is as wide as 20 degrees or wider, the vignetting factor due to increase of vignetting becomes 50% or less.
It is not impossible to design a projection lens with a characteristic having vignetting factor of 100% or higher without producing the vignetting. However, this will increase the size of the lens itself and increase manufacturing cost accordingly. Therefore, using a projection lens telecentric toward the organic EL element does not necessarily mean effectively projecting light emitted from the organic EL element and displaying a bright image.
Furthermore, when the organic EL element is caused to emit light continuously at high brightness, the brightness deteriorates notably. There are various causes for this, but one of major causes is that the current supplied to drive the organic EL element produces heat and when this heat is accumulated, the temperature of the element increases, which changes the structure or characteristic of the organic thin film and the luminance efficiency deteriorates gradually.
Thus, the organic EL element is expected to have improved durability through the development of materials at lower cost and improvements in the structure and driving method, etc.
However, when the organic EL element is used as an image modulation light source for the projection type image display apparatus, it is not necessary to widen the angle of visibility to make the organic EL element directly visible from all directions, and not all isotropic light beams emitted from the organic EL element are required, either. Thus, it is necessary to increase the amount of light captured by the projection leans and projected onto a diffusive object such as a screen. It is therefore possible to increase the ratio of light emitted which is captured by the pupil of the projection lens and reduce the ratio of unnecessary emitted light.
Thus, adopting a structure that consequently requires only a small amount of light emitted from the organic EL element can reduce the amount of charge carriers injected into the organic EL element, reduce the amount of heating of the element, suppress variations in the structure or characteristic of the organic thin film and slow down the speed at which the luminance efficiency deteriorates.
On the other hand, as the one capable of improving only directivity of light emitted from the organic EL element, Japanese Patent Application Laid-Open No. 2000-277266 discloses the following proposal. That is, when the organic EL element is used as a flat even illumination light source, the spread of directivity of light emission is suppressed using a prism layer called a “light condensing layer” to improve the directivity of light emission.
However, when the organic EL element is used as an image modulation light emission source, if there is no positional correlation between pixels making up the image and the prism layer, directivities of light emission are not unified into a certain direction. Furthermore, when the luminescent layer is distant from the prism layer, optical crosstalk occurs between pixels, which causes the image to be displayed as multiple images or deteriorates contrast.
Furthermore, when the organic EL element is used as a direct-view type image modulation light emission source, that is, in a system in which light directly enters the eyes from a light source such as a direct-view type display, head up display, head mounted display, etc., a brightness distribution in the light emission direction by a microprism becomes discontinuous, which produces variations in brightness and causes the image to be displayed with glare.
Furthermore, in the actual technological stage, light emission brightness of the organic EL element itself is still insufficient to be used for a display apparatus for enlarged projections when an element with a single-plate two-dimensional structure of three primary colors RGB (red, green, blue) is used. For this reason, it is more realistic to use a method whereby modulations of RGB colors are handled by their respective organic EL elements, three RGB colors are combined to compensate for the amount of light projected. This requires a dichroic wavelength-combining prism having a dichroic waveband separating film (hereinafter referred to as “dichroic film”) to combine RGB three primary color beams (color-combination).
Thus, a cross dichroic prism with a crisscross arrangement of dichroic films is most effective and generally used as a dichroic wavelength-combining prism having a projection lens with shorter back focus, which is effective in increasing brightness of the projection lens and widening the angle of projection.
However, since the organic EL element emits light without directivity by recombination of excitons, the emission characteristic of light emitted out of the film structure which forms the organic EL element has a spherical emission characteristic from the emission plane of the element. For this reason, when beams are combined using a cross dichroic prism with conventional dichroic films orthogonally arranged, the deflecting plane of which has a square section, the light emitted from the emission plane of the organic EL element in an oblique direction is partially reflected on neighboring mutually orthogonal planes of incidence of the cross dichroic prism, enters the projection lens and is likely to be displayed outside the image projection region of an object such as a screen as a ghost image.
Examples of remedial actions for this problem include application of black painting or diffusion treatment to the prism surface or coating of an anti-reflection film to areas that cannot be painted or treated. However, when light is reflected on the neighboring mutually orthogonal planes of the prism body, the angle of incidence is shallow with respect to the plane, and therefore reflectance on the interface is high and treatment of the interface cannot achieve significant ghost prevention effects.
On the other hand, as disclosed in the aforementioned Japanese Patent Application Laid-Open No. 1999-67448, even when directivity of a light emission characteristic is enhanced by a light resonance structure made up of a metal electrode in the organic EL element structure and the multilayer dielectric interference reflecting mirror placed outside the transparent electrode facing the metal electrode, it is characterized in that the emission wavelength varies depending on the emission direction and the half-value angle of emission intensity ranges approximately 30° to 40° at a half angle. Thus, even if a F number of the projection lens is set to approximately 1.5, the angle of incidence captured by the projection lens is a little less than 20°, which does not provide directivity enough to avoid reflections on the neighboring mutually orthogonal planes of the prism.